This invention relates to the regeneration of spent metal-sulfur compounds, and more particularly, to a process and apparatus for simultaneously generating and recovering elemental sulfur and re-usable metal oxide from spent metal sulfide sorbents.
A method and system for removal of sulfur compounds from gases and for regenerating spent sorbents is described in U.S. Pat. No. 4,857,285 which is incorporated by reference herein in its entirety. In U.S. Pat. No. 4,857,285, sulfur compounds in a hot gas moving in a direction which is countercurrent to the direction of movement of a movable bed of metal oxide, react with the metal oxide to form metal sulfide. The metal sulfide is regenerated to re-usable metal oxide in a moving bed regenerator, and the regeneration is carried out with an oxygen-containing gas in which regenerator off-gas serves as a diluent to control oxygen concentration. In the regenerator of U.S. Pat. No. 4,857,285, spent metal sulfide moves progressively through a single regeneration vessel having first, second and third regeneration stages, and in the first and second regeneration stages, first and second oxygen and sulfur dioxide-containing gases are moved, respectively, in a direction which is co-current with the direction of movement of the movable bed of spent metal sulfide. In the third regeneration stage, an oxygen-containing gas moves in a direction which is countercurrent to the direction of movement of the movable bed of metal sulfide. The combined gases which are rich in sulfur dioxide and lean in oxygen, derived from the first, second and third regeneration stages are removed from the regenerator as off-gas and used as diluent with air, oxygen-enriched air or pure oxygen to provide the appropriate oxygen concentration in the oxygen-containing gases introduced into the first and second regeneration stages.
In the process and apparatus disclosed in U.S. Pat. No. 4,857,285, regeneration of the sorbent material produces sulfur dioxide. The sulfur dioxide so recovered eventually has to be converted into an environmentally safe disposable product or into a usable chemical by-product. Frequently, these conversions require additional plant operations which result in added capital and operating costs to the plant.
In other desulfurization processes, spent sorbent materials are regenerated to produce elemental sulfur. For example, where iron sulfide is the spent sorbent, it is oxidized by controlled amounts of oxygen to produce elemental sulfur and ferric oxide according to equation (I): EQU 4 FeS+30.sub.2 .fwdarw.2 Fe.sub.2 O.sub.3 +4S (I)
Excess oxygen would further oxidize sulfur to sulfur dioxide. Elemental sulfur can be sold directly as recovered and is an environmentally safe product as compared with sulfur dioxide. Accordingly, it can be seen that a process for regenerating spent sorbents or an apparatus for regenerating spent sorbents which produces elemental sulfur instead of sulfur dioxide, is advantageous.
The desirability of higher elemental sulfur yield from the regeneration of sulfided or spent sorbents with oxygen-containing gases is discussed in U.S. Pat. No. 4,363,790, which is incorporated by reference herein in its entirety. In U.S. Pat. No. 4,363,790, sulfided compounds are formed from oxides of chromium, a combination of oxides of zinc and chromium, a combination of oxides of zinc and aluminum and mixtures thereof and are regenerated by contacting them with an oxygen-containing gas stream at temperatures of about 760.degree. C. (1400.degree. F.) to 1315.degree. C. (2400.degree. F.). Sulfur removed in the regeneration is in the form of elemental sulfur. In U.S. Pat. No. 4,363,790, it is indicated that the elemental sulfur may be increased by recycle of the regenerator off-gas and that the oxygen-containing gas used for regeneration may be any gas containing significant amounts of oxygen.
In view of the foregoing, it can be seen that it is desirable to provide improved processes and improved apparatus for regenerating sulfided or spent sorbents which result in a sulfur-containing off-gas with sulfur compound distribution favorable for recovery as elemental sulfur and to provide a process for the regeneration of spent metal-sulfur compounds which will produce elemental sulfur in the regenerator itself and to provide an apparatus for the regeneration of spent metal-sulfur sorbents which will produce elemental sulfur in the regenerator itself. It can also be seen that it is advantageous to provide such improvements in processes and apparatus already available and known in the art.